In one manufacturing process, lightguide fiber having a diameter of about 0.013 cm is drawn from a vertically suspended, heated glass preform and is taken up on spools. The spools are used to supply subsequent operations, such as ribboning in which a plurality of fibers are arrayed contiguously in a planar array. To satisfy system demands, lightguide fiber must have certain attributes, one of which is relatively high strength. It is essential that the strength of the fiber be preserved during and after the drawing operation.
The preservation of the strength of the fiber is accomplished by the application of a relatively thin layer of an organic or an inorganic coating material immediately after drawing. This coating which has a thickness of about 0.005 cm serves to prevent airborne particles from impinging upon and adhering to the surface of the drawn fiber which would weaken it. Also, the coating shields the fiber from surface damage which would be inflicted during subsequent manufacturing process and during installation.
It is important in the production of a drawn fiber that the coating process provide a coating having required properties. Important properties relating to the coating are its thickness, which determines the diameter of the coated fiber, its centering or concentricity, and the proper curing of the material which comprises the coating. An off-centered fiber in the coating may not adequately cover or protect the fiber surface which could have an adverse effect on fiber strength and microbending loss.
The thickness of the coating is important not only from the standpoint that it is sufficient to adequately cover and protect the surface of the fiber, but also that it is not so thick that it impairs subsequent manufacturing operations and/or connectorization. Too large a coated fiber diameter will cause the coated fiber undesirably to adhere to a subsequently tube extruded jacket of a single lightguide fiber cable. Also, in ribboning, fibers having an excessively thick coating will cause lateral displacement of contiguous fibers thereby unduly stressing the fibers as well as causing misalignment of those fibers in the array with fibers of another array during connectorization.
In one apparatus, fibers are coated as they are passed downwardly vertically from a drawing furnace and through an applicator containing a viscous liquid coating material. As the liquid coating material is drawn from the applicator by the moving fiber, it is replenished from an elevated reservoir by actuation of a valve in response to sensed levels of the coating material.
The drawing process as well as a coating process are disclosed in an article by D. H. Smithgall and D. L. Myers entitled "Drawing Lightguide Fiber" which appeared on pages 49-61 of the Winter 1980 issue of The Western Electric Engineer and which is incorporated by reference hereinto. Coating applicators are disclosed in U.S. Pat. No. 4,246,299 and in commonly assigned application Ser. No. 265,713 filed May 20, 1981 in the name of Rama Iyengar.
A typical coating applicator includes a die which depends from a reservoir cup having an open top and through which the drawn fiber is advanced. The diameter of the orifice of the die, which generally has been made of a relatively flexible material, is fixed at approximately two times the outer diameter of the uncoated fiber.
These are problems associated with the use of the fixed size orifice for applying a coating material on a drawn fiber. Unfortunately, the fixed size die does not permit adjustments to compensate for variables such as the level of the coating material in its applicator cup, changes in line speed, viscosity of the coating material, and wear or inaccuracies of die manufacture. Should the draw speed of the fiber be changed, the orifice in the die can become quickly flooded or starved. This has been a limiting factor on the utility of this type of coating apparatus where different coating materials and different draw speeds are contemplated. Also, the die cannot be easily cleaned or made serviceable without stopping the drawing operation.
Another problem comes about during the start-up of the drawing of the lightguide fiber from the preform which is suspended vertically above the coating die and a drawing furnace. During start-up with a fixed orifice die, an operator strings up the draw apparatus by pulling a lower portion of the preform downwardly through the furnace and threading it through measuring devices and through the coating apparatus. The portions of the preform which are strung up by the operator are at an elevated temperature and are enlarged relative to the drawn fiber. They cause the tip of the die to melt and form an unduly large opening. As a result, the amount of coating material which is applied to the fiber is greater than that desired.
As for the prior art, Thayer and Martin in their U.S. Pat. No. 19,316 show an article being moved through a wiper which is surrounded by a rubber strap having wraps displaced longitudinally along the article. The elasticity of the strap when tightly drawn causes it to closely contact the wiper, always fitting it to the article moving therethrough and wiping off surplus coating. Such an arrangement is completely unsuitable for coating lightguide fiber since the strap tends to twist or skew the wiper as it is pulled taut. This tends to deform the inner diameter inasmuch as the forces are not applied evenly and circumferentially around the periphery of the fiber at any point along its length.
A need remains for methods and apparatus for coating lightguide fibers by which the diameter of the coated fiber can be maintained within a range while the previously mentioned variables are changing. Seemingly this need has not been addressed by the prior art.